Lead-acid batteries are characterized as being inexpensive and highly reliable. As such, they are widely used as an electrical power source for starting motor vehicles, golf carts, and other electric vehicles. In recent years, a variety of measures to improve fuel efficiency have been considered in order to prevent atmospheric pollution and global warming. Examples of motor vehicles subjected to fuel-efficiency improvement measures that are being considered include idling stop vehicles (ISS vehicles) where the engine is stopped when the vehicle is not in motion to prevent unnecessary idling of the engine and to reduce engine operation time.
In an ISS vehicle, the number of engine startup cycles is higher, and the lead-acid battery discharges a large electrical current during each startup. In addition, the amount of electricity generated by the alternator in an ISS vehicle is smaller, and the lead-acid battery is charged in an intermittent manner. As such, charging of the battery is often insufficient. Stated differently, the battery is in a partially charged state known as a PSOC (i.e., partial state of charge). Accordingly, a lead-acid battery used in an ISS vehicle is required to have a capability in which the battery is charged as much as possible in a relatively short time. In other words, the lead-acid battery should have a higher charge acceptance. Therefore, improvements in the charge acceptance of a lead-acid battery are desired.
Lead-acid batteries typically have a shorter lifespan when used under PSOC than in an instance in which the battery is used in a fully charged state. One reason for the shorter lifespan under PSOC is believed to be due to repeatedly charging and recharging the battery in an insufficiently charged state. Charging and recharging the battery in this manner negatively affects the battery's electrodes or plates. For example, lead sulfate forms on the negative plate during discharge and undergoes progressive coarsening during charging and tends not to return to metallic lead. Improving the charge acceptance may prevent the battery from being charged and recharged in an insufficiently charged state, which may inhibit coarsening of lead sulfate due to repeated charging/discharging. This may increase the life span of the lead-acid battery.
In addition, there are inherent disadvantages to lead-acid batteries. For example, during discharge of the lead-acid battery, the lead dioxide (a fairly good conductor) in the positive plate is converted to lead sulfate (an insulator). The lead sulfate can form an impervious layer encapsulating the lead dioxide particles which limits the utilization of lead dioxide often to less than 50 percent of capacity, and more commonly around 30 percent. The low percentage of usage is a key reason why the power and energy performance of a lead-acid battery is inherently less than optimum. It is believed that this insulator layer leads to higher internal resistance for the battery. Improving the charge acceptance may also help reduce issues associated with formation of lead sulfate. In addition, lead-acid batteries having a separator typically exhibit a voltage drop when operated in cranking cycles at low operating temperatures (multiple starting procedures). This disadvantage hinders the acceptance of such battery systems for a broader use.